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Question 1Chemical Engineering

In a shell-and-tube heat exchanger, which quantity is best described by the log-mean temperature difference (LMTD)?

  1. The driving force for heat transfer averaged over the exchanger length Correct answer.
  2. The maximum temperature difference between the two streams anywhere in the unit
  3. The temperature rise of the hotter stream divided by its inlet temperature
  4. The arithmetic average of the two terminal temperature differences

Explanation

For counter-current or co-current flow with constant heat capacities, the local temperature difference varies exponentially along the exchanger, so the length-averaged driving force is the logarithmic mean of the terminal differences. The LMTD therefore represents the effective average driving force, corrected by F for multipass arrangements.

Question 2Chemical Engineering

A first-order irreversible liquid-phase reaction takes place in a plug-flow reactor. If the volumetric flow rate is doubled while temperature and feed concentration stay constant, the conversion for a fixed reactor volume will:

  1. Decrease Correct answer.
  2. Become zero
  3. Increase
  4. Remain unchanged

Explanation

For a first-order PFR, conversion depends on the Damkohler number k*V/v. Doubling the volumetric flow rate halves the residence time, so the Damkohler number halves and conversion falls.

Question 3Chemical Engineering

In distillation, the minimum reflux ratio corresponds to which condition?

  1. Total reflux with no product withdrawal
  2. An infinite number of equilibrium stages at the specified separation Correct answer.
  3. The operating line touching the equilibrium curve at a single point only at the reboiler
  4. A pinch at the feed stage with a finite stage count

Explanation

As reflux ratio is reduced toward its minimum, the operating and equilibrium lines approach each other and the required number of stages grows without bound. The minimum reflux condition is therefore defined by an infinite stage requirement, often with a pinch zone.

Question 4Chemical Engineering

Which dimensionless group represents the ratio of inertial to viscous forces in pipe flow?

  1. Froude number
  2. Nusselt number
  3. Prandtl number
  4. Reynolds number Correct answer.

Explanation

The Reynolds number is defined as rho*v*D/mu, which compares inertial stresses with viscous stresses and determines whether flow is laminar or turbulent. The other groups compare momentum to thermal diffusivity, convective to conductive heat transfer, and inertia to gravity respectively.

Question 5Chemical Engineering

A pump delivers 10 m3/h against a system head of 20 m. Taking water density as 1000 kg/m3 and g as 9.81 m/s2, the hydraulic power delivered to the fluid is closest to:

  1. 1.10 kW
  2. 5.50 kW
  3. 2.20 kW
  4. 0.55 kW Correct answer.

Explanation

Hydraulic power equals rho*g*Q*H. With Q = 10/3600 = 2.778e-3 m3/s, the product is 1000*9.81*2.778e-3*20, about 545 W, i.e. roughly 0.55 kW.

Question 6Chemical Engineering

In a packed absorption column, the flooding point is best characterised as:

  1. The gas rate at which the outlet gas reaches equilibrium with the inlet liquid
  2. The gas velocity above which liquid can no longer drain downward and accumulates in the bed Correct answer.
  3. The liquid rate at which the packing first becomes fully wetted
  4. The point where the pressure drop across the bed becomes zero

Explanation

At flooding, upward gas drag balances the downward liquid flow, so liquid holdup rises sharply and the column cannot operate stably. Design gas velocities are normally kept well below this limit to avoid excessive pressure drop and entrainment.

Question 7Chemical Engineering

Which of the following is the standard definition of the effectiveness of a heat exchanger?

  1. Overall heat transfer coefficient divided by the maximum coefficient
  2. Actual heat transfer rate divided by the maximum possible heat transfer rate Correct answer.
  3. Number of transfer units divided by the capacity ratio
  4. Log-mean temperature difference divided by the maximum temperature difference

Explanation

Effectiveness is the ratio of the heat actually transferred to the thermodynamic maximum, which is set by the smaller heat capacity rate and the inlet temperature difference. It lies between zero and one and links to NTU and capacity ratio through standard charts.

Question 8Chemical Engineering

In a binary mixture, the relative volatility alpha is defined as the ratio of the K-values of the two components. If alpha is exactly 1.0 at a given condition, the mixture:

  1. Forms an azeotrope at that composition and condition Correct answer.
  2. Behaves as an ideal solution obeying Raoult's law
  3. Has a bubble point equal to the dew point of the pure heavier component
  4. Can be separated by ordinary distillation with a very tall column

Explanation

When alpha equals one, vapour and liquid compositions coincide, so no enrichment is possible by simple distillation. That condition marks an azeotrope, requiring special methods such as extractive or pressure-swing distillation.

Question 9Chemical Engineering

A reaction A to B is exothermic and reversible. Increasing the reactor temperature while holding residence time constant will:

  1. Have no effect on either rate or equilibrium
  2. Raise the rate constant but lower the equilibrium conversion Correct answer.
  3. Lower the rate constant and lower the equilibrium conversion
  4. Raise both the rate constant and the equilibrium conversion

Explanation

Arrhenius kinetics mean the forward rate constant grows with temperature, but for an exothermic reversible reaction the equilibrium constant falls as temperature rises. The optimum temperature policy therefore balances kinetic gain against equilibrium loss.

Question 10Chemical Engineering

For fully developed laminar flow of a Newtonian fluid in a circular tube, the Fanning friction factor is best given by:

  1. 0.046/Re^0.2
  2. 64/Re
  3. 16/Re Correct answer.
  4. 0.079/Re^0.25

Explanation

The Hagen-Poiseuille result for laminar tube flow gives a Darcy friction factor of 64/Re, and the Fanning factor is one quarter of that, i.e. 16/Re. The other expressions are turbulent correlations.

Question 11Chemical Engineering

A single-effect evaporator concentrates a solution whose boiling point elevation is significant. Compared with water at the same pressure, the required steam temperature for the same heat transfer area will be:

  1. Independent of boiling point elevation
  2. The same
  3. Lower
  4. Higher Correct answer.

Explanation

Boiling point elevation raises the solution boiling temperature at a given pressure, reducing the temperature difference between condensing steam and boiling liquid. To maintain the same driving force and duty, a higher steam temperature is needed.

Question 12Chemical Engineering

In a drying operation, the critical moisture content marks the transition between:

  1. Constant-rate and falling-rate drying periods Correct answer.
  2. Bound moisture and free moisture removal
  3. Initial heating and constant-rate drying
  4. Equilibrium moisture and hygroscopic moisture

Explanation

While the surface remains saturated, drying proceeds at a constant rate governed by external heat and mass transfer. Once the critical moisture content is reached, the surface begins to dry and the rate falls as internal diffusion controls.

Question 13Chemical Engineering

A centrifugal pump is operated at constant impeller diameter with its speed doubled. According to the affinity laws, the head developed at the corresponding operating point becomes:

  1. Unchanged
  2. Half the original head
  3. Twice the original head
  4. Four times the original head Correct answer.

Explanation

For geometrically similar operation at constant impeller diameter, head scales with the square of rotational speed while flow scales linearly. Doubling the speed therefore quadruples the head.

Question 14Chemical Engineering

In a counter-current shell-and-tube exchanger, a correction factor F below 1.0 applied to the LMTD indicates that:

  1. The overall heat transfer coefficient has decreased
  2. The true mean driving force is lower than the pure counter-current LMTD Correct answer.
  3. The exchanger is operating above its design duty
  4. The flow arrangement is truly counter-current

Explanation

Multipass and cross-flow arrangements deviate from ideal counter-current behaviour, so the effective mean temperature difference is smaller than the counter-current LMTD. The factor F accounts for this reduction and is applied as F times LMTD.

Question 15Chemical Engineering

In liquid-liquid extraction, the distribution coefficient of a solute is 5 in favour of the solvent phase. For a single equilibrium stage with equal volumetric flow rates of feed and solvent, the fraction of solute remaining in the raffinate (on a solute-free basis, dilute system) is closest to:

  1. 5/6
  2. 1/2
  3. 1/6 Correct answer.
  4. 1/5

Explanation

For one equilibrium stage with equal phase flow rates, the fraction remaining in the raffinate is 1/(1 + m*S/F) with m = 5 and S/F = 1, giving 1/6. This follows from a solute balance combined with the equilibrium relation.

Question 16Chemical Engineering

Which statement about the adiabatic saturation temperature and the wet-bulb temperature of an air-water vapour mixture is correct?

  1. The wet-bulb temperature is always higher
  2. The adiabatic saturation temperature is always higher
  3. They differ by the latent heat of vaporisation
  4. They are essentially equal for air-water systems Correct answer.

Explanation

For air-water vapour mixtures the Lewis relation holds closely, so the wet-bulb temperature measured by a thermometer with a wetted wick nearly equals the adiabatic saturation temperature. For other gas-liquid pairs the two can differ appreciably.

Question 17Chemical Engineering

A second-order irreversible reaction with rate constant k occurs in a mixed flow (CSTR) reactor with space time tau. For equimolar feed of two reactants, the conversion is obtained from which relationship?

  1. tau = X^2/(k*C_A0*(1-X)^2)
  2. tau = X/(k*C_A0*(1-X)^2) Correct answer.
  3. tau = -ln(1-X)/k
  4. tau = X/(k*C_A0*(1-X))

Explanation

For a CSTR the design equation sets tau equal to conversion divided by the rate evaluated at the outlet conditions. With two reactants of equal concentration the rate is k*C_A0^2*(1-X)^2, giving the stated expression.

Question 18Chemical Engineering

In a binary distillation column, the feed is a saturated liquid. On the McCabe-Thiele diagram, the q-line is:

  1. Horizontal
  2. At 45 degrees
  3. Coincident with the equilibrium curve
  4. Vertical Correct answer.

Explanation

The q-line slope is q/(q-1). For a saturated liquid q equals one, so the slope is infinite and the q-line is vertical through the feed composition on the diagonal.

Question 19Chemical Engineering

In a shell-and-tube exchanger, adding segmental baffles primarily:

  1. Convert the flow to true counter-current operation
  2. Eliminate fouling on the tube side
  3. Increase shell-side velocity and heat transfer coefficient at the cost of higher pressure drop Correct answer.
  4. Reduce the tube-side pressure drop

Explanation

Segmental baffles force the shell-side fluid across the tube bundle, raising velocity and turbulence and thus the shell-side coefficient. The penalty is increased shell-side pressure drop and possible dead zones.

Question 20Chemical Engineering

A gas stream containing a soluble component is to be scrubbed. For a fixed liquid flow rate, raising the operating pressure in a packed absorber while keeping the gas molar flow constant will generally:

  1. Have no effect on the equilibrium solubility
  2. Decrease the absorption rate because diffusivity falls
  3. Increase the equilibrium partial pressure of the solute over the solvent
  4. Increase the liquid-phase solubility driving force and reduce the required liquid rate Correct answer.

Explanation

Higher total pressure raises the solute partial pressure for a given mole fraction, increasing its solubility according to Henry's law. This improves the driving force and can reduce the solvent circulation required for a target recovery.

Question 21Chemical Engineering

In a fluidised bed, the minimum fluidisation velocity is reached when:

  1. The upward drag force on the particles equals their buoyed weight Correct answer.
  2. The voidage of the bed falls to the packed-bed value
  3. The bed pressure drop becomes zero
  4. The particles begin to elutriate from the column

Explanation

At minimum fluidisation the pressure drop across the bed times its cross-section balances the effective weight of the solids. This condition is used with the Ergun equation to predict the minimum fluidisation velocity.

Question 22Chemical Engineering

A first-order liquid-phase reaction with rate constant 0.5 per minute is carried out in two CSTRs of equal volume in series, each with space time 2 minutes. The overall conversion is closest to:

  1. 0.90
  2. 0.50
  3. 0.67
  4. 0.80 Correct answer.

Explanation

For each CSTR of space time 2 min and k = 0.5/min, the outlet-to-inlet ratio is 1/(1 + k*tau) = 1/2. Two stages in series give (1/2)^2 = 1/4 remaining, so conversion is 0.75; with k*tau = 0.5 each stage gives 1/1.5, and squaring yields 0.444 remaining, i.e. about 0.56.

Question 23Chemical Engineering

Consider a binary distillation column separating an ideal mixture with constant relative volatility of 2.5. At total reflux, the minimum number of ideal stages for a separation from x_D = 0.95 to x_B = 0.05 (including the reboiler) is closest to:

  1. 6 Correct answer.
  2. 4
  3. 12
  4. 9

Explanation

Fenske's equation gives N_min = ln[(x_D/(1-x_D))*((1-x_B)/x_B)]/ln(alpha). The composition term is ln[(0.95/0.05)*(0.95/0.05)] = ln(361), about 5.89, and dividing by ln(2.5), about 0.916, gives roughly 6.4 stages, so about 6 to 7 including the reboiler.

Question 24Chemical Engineering

A hot process stream must be cooled from 150 to 90 degrees Celsius by a cold stream entering at 30 degrees Celsius. In a single-pass counter-current exchanger, the cold stream leaves at 120 degrees Celsius. The LMTD is closest to:

  1. 60 degrees Celsius
  2. 43 degrees Celsius Correct answer.
  3. 30 degrees Celsius
  4. 52 degrees Celsius

Explanation

In counter-current flow the terminal differences are 150-120 = 30 and 90-30 = 60. The LMTD is (60-30)/ln(60/30) = 30/0.693, about 43 degrees Celsius.

Question 25Chemical Engineering

A catalyst pellet is diffusion-limited for a first-order reaction. If the pellet size is halved while the intrinsic rate constant and surface concentration remain unchanged, the effectiveness factor will:

  1. Decrease by a factor of two
  2. Increase toward unity Correct answer.
  3. Remain unchanged
  4. Decrease by a factor of four

Explanation

The Thiele modulus for a first-order reaction scales with pellet dimension, so halving the size halves the modulus. In the strong-diffusion regime the effectiveness factor is inversely proportional to the modulus, so it roughly doubles and moves closer to unity.

Question 26Chemical Engineering

For a first-order irreversible reaction carried out in a plug flow reactor, the conversion obtained at a given space time is determined primarily by which dimensionless group?

  1. The Prandtl number of the reacting mixture
  2. The Reynolds number of the feed
  3. The Biot number of the reactor wall
  4. The Damkohler number k times tau Correct answer.

Explanation

In an ideal PFR the design equation integrates to conversion depending only on the product of the rate constant and space time, k times tau, which is the Damkohler number. This single group fixes conversion for first-order kinetics regardless of the absolute values of k and tau. Transport dimensionless groups do not enter ideal reactor design equations.

Question 27Chemical Engineering

In a shell-and-tube heat exchanger, which quantity is used to compare the actual heat duty with the maximum possible duty for given inlet conditions?

  1. The number of tube passes
  2. The fouling factor of the tubes
  3. The pressure drop across the shell
  4. The effectiveness of the exchanger Correct answer.

Explanation

Effectiveness is defined as the ratio of actual heat transferred to the thermodynamic maximum for the given inlet temperatures and heat capacity rates. It allows rating of any exchanger configuration without computing LMTD. Fouling factor, pressure drop and pass count affect performance but are not that ratio.

Question 28Chemical Engineering

For an ideal binary mixture obeying Raoult's law, the K-value of a component in a vapour-liquid equilibrium calculation is given by which expression?

  1. Mole fraction in liquid divided by mole fraction in vapour
  2. Total pressure divided by vapour pressure
  3. Activity coefficient divided by total pressure
  4. Vapour pressure divided by total pressure Correct answer.

Explanation

Raoult's law gives the partial pressure as the liquid mole fraction times the pure-component vapour pressure, so the ratio of vapour to liquid mole fraction equals vapour pressure over total pressure. That ratio is the K-value. The other expressions invert or misplace the quantities.

Question 29Chemical Engineering

A Newtonian liquid flows through a horizontal pipe at steady state with no heat transfer. Which quantity remains constant along the pipe if the fluid is incompressible?

  1. The volumetric flow rate Correct answer.
  2. The kinetic energy per unit mass
  3. The average velocity gradient at the wall
  4. The static pressure

Explanation

For steady incompressible flow in a constant-area pipe, continuity requires the volumetric flow rate to be constant everywhere. Static pressure falls because of friction, and the wall velocity gradient and kinetic energy are not conserved quantities along the pipe.

Question 30Chemical Engineering

In a distillation column operating at total reflux, what is the minimum number of theoretical stages related to?

  1. It is the number of stages needed at the economic optimum reflux ratio
  2. It equals the actual number of trays divided by the overall efficiency
  3. It corresponds to the minimum reflux ratio obtained from the pinch point
  4. It corresponds to the minimum stages at total reflux found by stepping between the equilibrium curve and the diagonal Correct answer.

Explanation

At total reflux the operating lines coincide with the diagonal of the McCabe-Thiele diagram, and stepping between equilibrium curve and diagonal gives the minimum theoretical stages. This is the limiting case of infinite reflux and finite stages. Minimum reflux and economic optimum are different limiting conditions.

Question 31Chemical Engineering

A slurry is to be filtered in a plate-and-frame press at constant pressure. The filtrate volume collected is plotted against time. For a cake with negligible cloth resistance, the plot of time divided by volume against volume is expected to be:

  1. A parabola opening upward
  2. A horizontal line
  3. A straight line with positive slope Correct answer.
  4. An exponential decay

Explanation

Constant-pressure filtration with negligible medium resistance follows the Ruth equation, in which time over volume is linear in volume. The slope reflects the specific cake resistance and the solids loading. Curvature would appear only if the assumptions such as constant pressure or incompressible cake fail.

Question 32Chemical Engineering

In a binary mixture, the dew point at a given pressure is the temperature at which:

  1. The latent heat of vaporisation becomes zero
  2. The two phases have identical composition
  3. The first drop of liquid forms from a vapour of fixed composition Correct answer.
  4. The last bubble of vapour disappears from a liquid of fixed composition

Explanation

The dew point of a vapour mixture is the temperature at which condensation begins as the mixture is cooled at constant pressure. The bubble point, by contrast, marks the appearance of the first vapour on heating a liquid. Identical phase compositions occur only at an azeotrope.

Question 33Chemical Engineering

Which dimensionless group is used to characterise the relative importance of convective and diffusive mass transfer in a flowing fluid?

  1. The Lewis number
  2. The Nusselt number
  3. The Schmidt number
  4. The Sherwood number Correct answer.

Explanation

The Sherwood number expresses the ratio of convective to diffusive mass transfer at a boundary and is the mass-transfer analogue of the Nusselt number. The Schmidt number compares momentum and mass diffusivity, and the Lewis number compares thermal and mass diffusivity.

Question 34Chemical Engineering

A gas is absorbed into a liquid in a wetted-wall column. The liquid film is laminar and the solute concentration at the interface is fixed. According to the penetration theory, the local mass-transfer coefficient varies with contact time as:

  1. Independent of contact time
  2. Proportional to the inverse square root of contact time Correct answer.
  3. Proportional to the square of contact time
  4. Proportional to contact time

Explanation

Penetration theory treats unsteady diffusion into a stagnant element, giving a flux proportional to the square root of diffusivity over time. Hence the coefficient scales as the inverse square root of contact time. The film theory, by contrast, predicts a coefficient independent of time.

Question 35Chemical Engineering

Two CSTRs of equal volume are arranged in series for a second-order reaction. Compared with a single CSTR of the same total volume, the series arrangement gives:

  1. Lower conversion for the same total space time
  2. Higher conversion for the same total space time Correct answer.
  3. The same conversion because total volume is unchanged
  4. Conversion that depends only on the inlet concentration

Explanation

Splitting the volume into stages raises the average reactant concentration seen by the second reactor, and for reaction orders above one this increases overall conversion. A single CSTR operates at the low outlet concentration throughout. This is why staging improves performance for positive-order kinetics.

Question 36Chemical Engineering

A liquid flows through a packed bed of spheres at a Reynolds number based on particle diameter of about 10. The pressure drop is best estimated using which relationship?

  1. The Fanning equation for a smooth pipe
  2. The Stokes drag law for a single sphere
  3. The Ergun equation combining viscous and inertial terms Correct answer.
  4. The Hagen-Poiseuille equation for a single tube

Explanation

The Ergun equation describes pressure drop through packed beds by summing a viscous Blake-Kozeny term and an inertial Burke-Plummer term, and it applies across the laminar and transitional regimes. At a particle Reynolds number of 10 the viscous term dominates but the combined form remains appropriate. Single-tube and single-sphere relations ignore the bed geometry.

Question 37Chemical Engineering

In a double-pipe heat exchanger the hot stream flows in the annulus and the cold stream in the inner tube in the same direction. If the hot inlet is 120 degrees Celsius and the cold inlet is 20 degrees Celsius, the terminal temperature difference at the cold inlet end is:

  1. Zero because the streams are parallel
  2. Undefined without knowing the flow rates
  3. 100 degrees Celsius Correct answer.
  4. The difference between the two outlet temperatures

Explanation

In parallel flow both streams enter at the same end, so the terminal difference at that end is the hot inlet minus the cold inlet, 120 minus 20, giving 100 degrees Celsius. The other terminal difference uses the two outlet temperatures. Flow rates affect the outlet temperatures but not this inlet-end difference.

Question 38Chemical Engineering

A distillation column separates a binary mixture with relative volatility 3.0. The feed is saturated vapour. On the McCabe-Thiele diagram, the q-line for this feed is:

  1. Horizontal at the feed composition
  2. Coincident with the diagonal
  3. Sloping with the equilibrium curve
  4. Vertical through the feed composition Correct answer.

Explanation

For a saturated vapour feed, q equals zero, and the q-line equation reduces to a vertical line at the feed mole fraction. A saturated liquid would give a vertical line as well, but a two-phase feed gives a sloped line. The diagonal corresponds to total reflux operating lines.

Question 39Chemical Engineering

A first-order reaction occurs in a spherical catalyst pellet where pore diffusion is significant. The effectiveness factor is defined as:

  1. Pellet volume divided by external surface area
  2. Surface concentration divided by centre concentration
  3. Actual reaction rate divided by the rate if the entire pellet were at surface conditions Correct answer.
  4. Pore diffusivity divided by bulk diffusivity

Explanation

The effectiveness factor compares the actual diffusion-limited rate with the rate that would prevail if the whole pellet experienced the surface concentration and temperature. It is always at most one for isothermal first-order kinetics. The Thiele modulus determines its value.

Question 40Chemical Engineering

A centrifugal pump delivers water at a certain head and flow rate. If the impeller diameter is increased by 10 percent at constant speed, the head developed changes approximately by which factor?

  1. 1.10
  2. 1.21 Correct answer.
  3. 1.33
  4. 1.00

Explanation

Affinity laws state that head varies with the square of impeller diameter at constant speed, so 1.10 squared gives about 1.21. Flow rate varies linearly and power with the cube. The other factors correspond to the wrong exponent.

Question 41Chemical Engineering

In a counter-current extraction cascade, the slope of the operating line on an equilibrium diagram is determined by:

  1. The number of equilibrium stages
  2. The interfacial area of the contactor
  3. The equilibrium distribution coefficient alone
  4. The ratio of solvent to feed flow rates Correct answer.

Explanation

A solute mass balance around the cascade gives an operating line whose slope is the ratio of solvent to carrier flow rates. The equilibrium curve supplies the distribution coefficient. The number of stages and interfacial area affect the height or extent of the cascade but not the operating-line slope.

Question 42Chemical Engineering

Air at 40 degrees Celsius dry-bulb and 25 degrees Celsius wet-bulb is heated without adding moisture. Which property remains essentially unchanged during the heating?

  1. The humidity Correct answer.
  2. The relative humidity
  3. The enthalpy of the air-water mixture
  4. The dry-bulb temperature

Explanation

Sensible heating of air with no moisture addition keeps the absolute humidity constant because no water is added or removed. Relative humidity falls and dry-bulb temperature rises, and the mixture enthalpy increases because sensible heat is supplied.

Question 43Chemical Engineering

A liquid is boiling in a vertical thermosiphon reboiler. As the heat flux is increased past the critical heat flux, what happens to the boiling regime?

  1. The liquid subcooling increases and boiling ceases
  2. The regime remains nucleate boiling with rising heat-transfer coefficient
  3. The surface tension effect reverses and bubbles stop forming
  4. The vapour blanket causes a transition to film boiling with a sharp drop in heat-transfer coefficient Correct answer.

Explanation

Beyond the critical heat flux, vapour cannot leave the surface fast enough and a stable vapour film forms, giving film boiling. The heat-transfer coefficient falls sharply because the vapour film has low thermal conductivity. This is the boiling crisis associated with burnout risk.

Question 44Chemical Engineering

A drying operation removes free moisture from a solid under constant drying conditions. During the constant-rate period, the drying rate is controlled mainly by:

  1. The internal diffusion of moisture through the solid
  2. The external heat and mass transfer at the surface Correct answer.
  3. The porosity of the dry solid
  4. The equilibrium moisture content of the solid

Explanation

In the constant-rate period the surface stays wet and the rate is set by external heat transfer and vapour diffusion from the surface into the air. Internal moisture migration becomes controlling in the falling-rate period. Equilibrium moisture content and porosity influence the extent of drying rather than the constant-rate flux.

Question 45Chemical Engineering

In a gas-liquid absorption column, the overall gas-phase mass-transfer coefficient is dominated by a very high Henry's law constant for the solute. This indicates that:

  1. The column operates at equilibrium
  2. Neither film offers resistance
  3. The liquid-film resistance controls the overall rate Correct answer.
  4. The gas-film resistance controls the overall rate

Explanation

A high Henry constant means the solute is sparingly soluble, so the equilibrium back-pressure in the gas is large and the liquid-film resistance dominates. The overall coefficient then approaches the liquid-side coefficient. A highly soluble gas would instead be gas-film controlled.

Question 46Chemical Engineering

A reaction A to B is carried out in a packed-bed reactor with strong exothermicity. Which phenomenon most directly causes the axial temperature profile to develop a hot spot?

  1. The pressure drop through the bed
  2. The axial dispersion of the reactant
  3. The balance between heat generation and heat removal along the bed Correct answer.
  4. The catalyst pellet porosity

Explanation

A hot spot forms when the local heat release from reaction exceeds the rate at which heat is conducted axially or removed to the wall. The interplay of generation and removal sets the temperature profile. Pressure drop, dispersion and porosity influence performance but are not the primary cause of the temperature peak.

Question 47Chemical Engineering

A Bingham plastic is sheared in a pipe. Which feature distinguishes its flow from that of a Newtonian fluid at low applied stress?

  1. It exhibits a yield stress and can remain rigid below that stress Correct answer.
  2. It expands in volume as shear rate rises
  3. It shows a viscosity that decreases with increasing shear rate indefinitely
  4. It has a viscosity independent of shear rate

Explanation

A Bingham plastic requires a minimum yield stress before it deforms, and below that stress it behaves as a rigid solid. Above the yield stress it flows with a constant plastic viscosity plus the yield contribution. Shear-thinning without a yield stress describes a power-law fluid instead.

Question 48Chemical Engineering

A binary distillation column is designed at a reflux ratio only slightly above minimum. What is the practical consequence for the required number of stages?

  1. The stage count becomes independent of reflux
  2. The stage count rises steeply toward infinity as reflux approaches minimum Correct answer.
  3. The stage count equals the number of actual trays
  4. The stage count falls toward the minimum stages at total reflux

Explanation

As reflux approaches the minimum, the operating and equilibrium lines pinch and the number of theoretical stages grows without bound. Operating slightly above minimum therefore requires many stages, which is why designs balance capital cost of stages against energy cost of reflux. Total reflux is the opposite limit.

Question 49Chemical Engineering

A gas stream is scrubbed in a packed tower, and the operating line crosses the equilibrium line within the column. The physical implication is:

  1. The driving force reverses and absorption cannot proceed past the crossing point Correct answer.
  2. The liquid rate must be reduced to fix the problem
  3. The packing wetted area doubles automatically
  4. The tower height becomes zero

Explanation

If the operating line crosses the equilibrium curve, the concentration driving force changes sign, so the solute would have to desorb rather than absorb beyond that point. This usually signals an inadequate liquid flow rate or an inappropriate solvent. Design requires the operating line to stay on the absorption side of equilibrium.

Question 50Chemical Engineering

In a jacketed stirred-tank reactor, the heat-transfer coefficient on the jacket side is much smaller than on the process side. To improve heat removal most effectively, one should:

  1. Reduce the agitator speed
  2. Increase jacket-side turbulence or use a higher-conductivity jacket fluid Correct answer.
  3. Thicken the vessel wall
  4. Polish the inner vessel wall

Explanation

When one film coefficient is much smaller, it dominates the overall resistance, so improving that side gives the largest gain. Increasing jacket-side turbulence or using a better-conducting fluid raises the limiting coefficient. Changes to the process side or wall have little effect while the jacket resistance dominates.